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What causes cancer?

Cancer cells growing in the labThere are many things that can cause cancer – and many things that people wrongly believe cause the disease. And for many cancers, we simply don’t know the cause.

The fundamental cause of cancer is damaged or faulty genes – the instructions that tell our cells what to do. Genes are encoded within DNA, so anything that damages DNA can increase the risk of cancer. But a number of genes in the same cell need to be damaged before it becomes cancerous.

Most cancers are caused by DNA damage that accumulates over a person's lifetime. Cancers that are directly caused by specific genetic faults inherited from a parent are rare. But we all have subtle variations in our genes that may increase or decrease our risk of cancer by a small amount.

So cancer risk isn't "all in the genes", and it's not all down to lifestyle - it's a combination of the two. We can't change our genetic makeup, but we can all take steps to reduce our risk of cancer by following healthy lifestyle advice based on scientific research.


Age is the single biggest risk factor for cancer – the older you are, the more likely you are to develop cancer. Nearly two-thirds of all cases of cancer diagnosed in the UK occur in people over 65 years old. This is because the longer we live, the more cancer-causing faults we accumulate in our DNA.

It also explains why more people are getting cancer nowadays. Thanks to advances in public health and the prevention of infectious diseases, we live much longer, increasing our chances of picking up cancer-causing DNA faults.

Read more about why cancer isn't only a modern disease on our Science Update Blog.


Up to half of all cancers could be prevented by changes to lifestyle, and there are many things we can do to try to reduce the risk of the disease.

These include giving up smoking, enjoying the sun safely, eating a healthy balanced diet, limiting alcohol, keeping physically active and sticking to a healthy bodyweight.

There is much more detailed information about lifestyle and cancer risk in our Healthy Living pages.

DNA damage

DNA damage is extremely common - some studies suggest that the DNA in a single human cell gets damaged over 10,000 times every day.

For a start, the life-sustaining chemical reactions that occur naturally in our cells generate harmful by-products, and these can cause DNA damage. So merely being alive leads to DNA damage and this can potentially cause cancer.

Also our everyday surroundings are full of things that constantly damage the DNA in our cells, known as carcinogens.

Although our cells are very good at repairing this damage, errors can accumulate over the years. This explains why cancer usually affects older people.

There is more information about DNA damage and repair elsewhere on our site, as well as highlights of the world-class research our scientists are doing in this area.


‘Carcinogen’ literally means ‘something that causes cancer’. Carcinogens damage DNA, causing faults in important genes that can lead to cancer. Examples include:

But it is a mistake to believe that exposure to carcinogens is the only cause of cancer. In almost all cases, carcinogens are contributing factors, but there is a whole host of other factors at work, including a person's lifestyle and genetic makeup.

Inherited gene faults

Some people are born with a fault in one of their genes. This does not mean that they will ever actually develop cancer, but it does mean that fewer other things need to go wrong with the rest of their DNA for the disease to develop.

For example, women born with a mutation in one of their BRCA genes have a much greater chance of developing breast and ovarian cancer than women who do not. Faults in a BRCA gene can also increase a man's risk of prostate cancer.

People with a strong family history of these cancers can go for genetic testing, to find out whether they carry the faulty gene. Those at risk may be offered prevention advice or treatment to help reduce their chances of getting cancer, or screening to detect it at an early stage.

Cancer Research UK scientists have played a major role in discovering some of the gene faults involved in cancer, and we continue to fund ground-breaking work in this area. And there’s more about genes and cancer on CancerHelp UK.


Some viruses are linked to certain types of cancer. This does not mean that these cancers spread from person to person like an infection - you cannot 'catch' cancer - and does not mean that everyone infected with these viruses will develop cancer.

Our researchers are working hard to understand how viruses can cause cancer, and why some people are susceptible to developing the disease while others aren't.

Some examples include:

There are examples of infectious tumours in dogs and Tasmanian Devils, where cancer cells can directly transfer from one animal to another. But there is no evidence for these kinds of tumours in humans. 

Find out more about how Cancer Research UK-funded scientist Professor Valerie Beral first discovered that KSV was linked to cancer.

Problems with the immune system

People who have problems with their immune system are at higher risk of developing cancer, probably because they are less able to combat infections by viruses that are linked to the disease (see above).

This group includes people who have had an organ transplant and are taking drugs to suppress their immune responses, as well as people with diseases that affect the immune system, such as HIV/AIDS.

People who are born with one of several rare genetic diseases that affect their immune system - such as ataxia telangiectasia or A-T - are also at higher risk of developing cancer.

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Question about cancer? Contact our information nurse team
Updated: 25 September 2009